## JuMP v0.21.2

[Diff since v0.21.1](jump-dev/JuMP.jl@v0.21.1...v0.21.2)


**Closed issues:**
- Empty sums (jump-dev#2120)
- Creating empty SparseAxisArray not possible (jump-dev#2169)
- Initialize JuMP model when optimizer is nothing (jump-dev#2172)
- Writing nonlinear expressions to file does not work (jump-dev#2180)
- Easy way to determine constraints and/or variables leading to failed solving? (jump-dev#2185)
- Problem with two empty summations in a constraint (jump-dev#2187)
- Variable x[1] does not belong to this model (jump-dev#2189)
- Model(Optimizer) produces an error (jump-dev#2192)
- missing add_to_expression! method (jump-dev#2197)
- write_to_file doesn't work on windows (jump-dev#2206)
- JuMPScalars should have `conj(x) = x` defined since they are required to be real (jump-dev#2208)

**Merged pull requests:**
- prep for 0.21.1 (jump-dev#2168) (@mlubin)
- Add @variable to (model, x) in anon var doc (jump-dev#2177) (@mlhetland)
- Fix sum of expr with scalar product (jump-dev#2178) (@blegat)
- Fix construction of empty SparseAxisArray (jump-dev#2179) (@blegat)
- Fix writing of NL models to MOF (jump-dev#2181) (@odow)
- Remove extra ` in docstrings (jump-dev#2182) (@daschw)
- Missing space. (jump-dev#2184) (@dourouc05)
- Add fallback for `_functionize(::MA.Zero)` (jump-dev#2188) (@odow)
- Mark SOC support as done (jump-dev#2193) (@mlubin)
- Added Base.empty! method for JuMP.Model. (jump-dev#2198) (@henriquebecker91)
- jump-dev#2158 Added relative_gap() to access MOI.RelativeGap() attribute (jump-dev#2199) (@vrngupta53)
- Fix missing docstring source links (jump-dev#2207) (@amontoison)
- added conjugation operator for scalars (jump-dev#2209) (@ExpandingMan)
- add docstring for variables macro (jump-dev#2216) (@Aks1996)
- prep for 0.21.2 (jump-dev#2218) (@blegat)

Merge pull request jump-dev#2168 from JuliaOpt/ml/0211

prep for 0.21.1

bump Project.toml for 0.21

See github.com/JuliaOpt/JuMP.jl/releases/tag/v0.20.1 for release notes

See github.com/JuliaOpt/JuMP.jl/releases/tag/v0.20.0 for release notes

See github.com/JuliaOpt/JuMP.jl/releases/tag/v0.19.2 for release notes

See github.com/JuliaOpt/JuMP.jl/releases/tag/v0.18.6 for release notes

Merge pull request jump-dev#1867 from JuliaOpt/bl/prep019

[ci skip] prep for 0.19.0

Prep for 0.19-beta2 (jump-dev#1753)

Needed because 0.19-beta is incompatible with MOI 0.8.1.